The goal of this proposal is to test the overall hypothesis that subjects with chronic or recurrent low back pain (LBP) have an abnormal soft tissue response to acupuncture needling, and that this altered response is associated with abnormal perimuscular connective tissue structure and biomechanics. We will also test whether or not altered needling responses in LBP are generalized, or localized to specific locations as predicted by traditional acupuncture theory. 80 LBP and 80 No-LBP subjects will undergo one testing session using computerized robotic acupuncture needling and ultrasound elastography. Needle torque, force and tissue displacement patterns will be compared in LBP vs. No-LBP at Meridians vs. Non-Meridians and Acupuncture Points vs. Non-Acupuncture Points in the back and leg (Aim 1). We also will evaluate soft tissue structural and biomechanical characteristics in the low back by measuring perimuscular connective tissue thickness, ultrasound signal to noise ratio, structural continuity and biomechanical parameters (stiffness and damping) (Aim 2). Accomplishing the Aims of this study will 1) advance our understanding of acupuncture mechanisms 2) provide a first step toward investigating a new dynamic pathophysiological model for LBP incorporating connective tissue and neuroplasticity and 3) provide objective outcome measures of connective tissue structure and biomechanics that can be used in future clinical trials of acupuncture and other therapies for LBP. Relevance: Despite its widespread use for the treatment low back pain (LBP), the mechanism by which acupuncture may promote healing in this condition remains largerly unknown. This lack of understanding constrains the development of improved treatments and is an obstacle to the integration of acupuncture into the mainstream management of LBP. Moreover, the mechanism of LBP itself is poorly understood, further impairing efforts to investigate treatment mechanisms. This study will provide new, objective measurements of phenomena fundamental to both acupuncture and LBP. Demonstrating that acupuncture needling responses are abnormal in LBP, and understanding why these responses are abnormal will bring us one step closer to understanding 1) why LBP occurs and 2) how acupuncture can promote healing in this common and disabling condition.